Polyamide resins are well known as a class of resins, as are numerous methods for their preparation. Polyamide resins are typically manufactured by reacting a di- or polyfunctional amine with a di- or polyfunctional acid. Most of the commonly-employed diacids and diamines yield polyamide resins which are essentially linear.
The properties of polyamide resins will vary considerably, depending upon the particular synthetic reactants employed. Polyamide resins which are prepared from relatively short chain diacids and diamines having, for example, 5-10 carbon atoms will tend to be relatively crystalline and have excellent fiber forming properties. These types of polyamide resins are typically referred to as nylons.
Polyamide resins are also prepared from relatively long chain polyfunctional acids and diamines. A particularly important class of polyamide resins of this type are referred to as polymerized fatty acid polyamide resins. The polymerized fatty acid polyamide resins are especially useful in products such as hot-melt adhesives, water-resistant coatings, and binders for printing inks, because of their physical properties, including high strength, excellent flexibility, water and solvent resistance, and the ability to form smooth, non-tacky coatings and films.
The polyfunctional acids used in the preparation of polymerized fatty acid polyamide resins are derived from higher molecular weight unsaturated fatty acids by polymerization. In the polymerization process, the fatty acids having double bond functionalities combine to produce mixtures of higher molecular weight polymeric acids. Saturated fatty acids can also, under certain reaction conditions, polymerize to form polycarboxylic acids.
The polymerized fatty acid polyamide resins are, in turn, typically prepared by reacting one or more suitable diamines (most commonly relatively short chain diamines) with the polymerized fatty acid. Often, another diacid is also reacted to increase the softening point, tensile strength, or other properties. The polymerized fatty acid polyamide resins which are obtained tend to be more amorphous than the nylon types of polyamides resins and are generally more flexible. The differences in the physical properties of the polymerized fatty acid polyamide resins as compared to the nylon types of polyamide resins are related to the long chain length and structural variations of the polymerized fatty acid component.
One of the issues associated with the use of polyamide resins, particularly the polymeric fatty acid polyamides, relates to the methods used to apply the resins to substrates. One method which has been used involves heating the polyamide resins above their melting point and then applying the molten resins to the substrate. This technique, however, has certain inherent problems. For example, polyamide resins typically have high melting points, often higher than the distortion temperatures of the substrates onto which they are to be applied. Accordingly, the hot melt method can only be used in certain limited applications which require relatively expensive application equipment.
It has been recognized that it would be advantageous if the polyamides could be applied at ambient temperatures as solutions or dispersions. For many applications, however, solutions of polyamide resins are unsatisfactory. Polyamide resins as a class have excellent resistance to solvents; even with respect to those solvents in which the polyamide resins are soluble, the solubility typically is relatively low. Furthermore, the solvents which have been used to make polyamide resin solutions often adversely react with the substrates to which the polyamide resin solutions are applied. Further problems associated with solvent solutions are that most solvents used are relatively expensive, often difficult or impossible to remove from the applied coatings, and present fire, toxicity, and environmental pollution problems.
To overcome or at least reduce the problems associated with such solvent-based systems, it has been suggested to prepare emulsions or dispersions of the polyamide resins in water. Early emulsions were prepared by initially dissolving the polyamide resin in an organic solvent and then using selected emulsification agents to form an emulsion of the solvent solution and water. However, the resulting solvent/water/polyamide resin emulsions still had the problems associated with the presence of solvents and were relatively unstable. Those skilled in the art will appreciate that instability is manifested in aqueous resin emulsions or dispersions by phenomena such as phase separation, creaming, coalescence, flocculation, or gelation. Films formed from solvent-containing emulsions also tended to have an undesirable tackiness.